I think that's the best practical reason I've heard for why this plan may not have been considered. The only potential problem with this theory is that the eagles DID fly over Mordor to rescue Frodo and Sam...BUT I'm thinking they may not have been sure if they COULD originally do so, and didn't want to take that chance while carrying the Ring. In the movies Elrond, Denethor, Haldir, Galadriel, and Celeborn stink.